Background Art
The natural joints in the human body are often subject to degenerative changes resulting, inter alia, from disease or trauma. If these defects have progressed too far so as to be irreversible and untreatable, it is necessary to replace the natural joints or bones by corresponding implants. Examples of bone replacement materials that may be mentioned are shaped implants of a wide variety of kinds. For implementation, these bone replacement materials may be anchored in the natural bone using bone cements.
Customary bone cements are composed of a solid component, which consists of a finely divided acrylic polymer and of further additives, such as polymerization initiators, together if desired, with X-ray contrast media and colorants, and of a liquid component, which consists of an acrylic monomer and of further additives such as polymerization accelerators and stabilizers. The polymer powder component of cement consists preferably of granular particles with a spherical shape. The particle size preferably lies within a narrow range or is substantially uniform.
A device for combining the polymer powder and the liquid monomer in a sterile manner is disclosed in commonly assigned U.S. Pat. No. 5,370,221 entitled "Flexible Package for Bone Cement Components", the disclosure of which is hereby incorporated by reference as if fully set forth herein. In one embodiment, the device has a first chamber for the liquid monomer, a second chamber for the powdered polymer, a removable barrier between the first and second chambers, a vacuum chamber, and a filter. The vacuum chamber of this device is in fluid connection with the second chamber; the filter prevents the powdered polymer from entering the vacuum chamber. Removal of the barrier permits fluid communication between the first and second chambers. The vacuum in the vacuum chamber draws the liquid monomer through the powdered polymer, permitting the two components to be precisely mixed at a predetermined ratio in a sterile environment. While this device made a significant advance in the provision of sterile bone cements, further improvement in the area of vacuum mixing is desirable.
For example, in situations where the vacuum mixing devices have been stored for a large period of time, it has been demonstrated that the vacuum level in the vacuum chamber of some of these devices tends to decrease over a period of time. When vacuum mixing devices that have been stored for long periods of time are used, the vacuum chamber may not have a sufficient level of vacuum to cause the liquid monomer to fully wet all of the powdered polymer. In such cases, it is then necessary to manually mix the components by kneading the material in the container.
One solution suggested to increase the shelf life of the device relates to an increased amount of vacuum by either enlarging the vacuum chamber or increasing the magnitude of the vacuum. However, if the level of vacuum is too great, a portion of the liquid monomer tends to be drawn completely through the powdered polymer and into the vacuum reservoir. In such situations, the powdered polymer and the liquid monomer are not mixed at their proper ratio.